Means for measuring difference in frequency of alternating currents



June 28, 1949. B. B. JAcoBsEN 2,474,253

MEANS FOR HEASURING DIFFERENCE IN FREQUENCY OF ALTERNATING CURRENTS Filed Feb. 24, 1944 2 Sheets-Sheet 1 @um mss B. B. JAcoBsEN v MEANS FOR IIEASURING DIFFERENQE IN FREQUENCY 0F ALTERNATING CURRENTS 2 Sheets-Sheet 2 June 28, 1949.

Filed Feb 24 1944 Patented June 28, 1949 LIEANS FOR MEASURING DIFFERENCE 1N FREQUENCY 0F ALTERNATING CUR- RENTS Bent Bulow Jacobsen, London, England, assignor,

by mesne assignments, to International Standard Electric Corporation, New York, N. YLT-a corporation of Delaware Application February 24, 1944, Serial No. ,$23,660. In Great Britain April 1, 1943 23 Claims. i

The present invention relates to electrical arrangements for measuring frequency difference, including also means for indicating the sign of the diiference.

There are a number of widely diifering cases where it is required to indicate or measure the difference between two frequencies for the purpose of deriving information or carrying out some function. In some of these cases also, it is necessary to be able to determine the sign of the difference. that is, it is required to know which of the frequencies is the higher. One application of interest, for example, is to systems for setting thecourse of aeroplanes, and

such a system will be employed to illustrate the invention. It should be understood, however, that the invention is of general interest and is not intended to be limited to any particular application.

The measuring or indicating circuits of this invention employ arrangements in which waves of one of the frequencies/are in eifect modulated by waves of the other. The processes involved can be equally regarded as modulation or demodulation" according to choice, there being no fundamental diiference between the two processes. Accordingly the term modulation or modulator will be used uniformly throughout this specification to indicate any combina.- tion, or means for combining, two frequencies to derive a sum and/or difference frequency there- The invention employs circuit arrangements which are known in the art as "single-sideband modulators, the features of which are explained later on in the specification. It will suffice at this point to state that single-sideband modulators can be made to have the property of suppressing the upper and/or the lower sideband under certain conditions by a cancellation process which does not involve the use of filters.

According to the invention, there is provided an electrical circuit for measuring in sign and magnitude the difference between the frequencies of two waves, comprising means for applying the -waves of both frequencies to each of two single-sideband modulators arranged in such a manner that a lower sideband is produced only in one modulator when the frequency difference has one sign and only in the other when it has the other sign.

According to another aspect, the invention consists in an electrical circuit for deriving an electric lcurrent or voltage depending in magnitude and sign on the dierence between the frebranches containing modulating elements and'k quadrature phase dierence networks arranged in such a manner that a sideband having the diierence frequency is obtained only in one branch when the diiference is positive, and only in the other branch when the difference is negative.

According to another view, the invention may be regarded as an electrical circuit for the determination of the sign and magnitude of the difference 4between two wave frequencies, comprising two single-sideband modulating branches which are so disposed that when supplied with waves of both the frequencies, substantially only a single lower sideband having the difference frequency is obtained, this single sideband being obtainedfrom one of the branches when the difference is positive, and from the other branch when the difference is negative.

According to afurther aspect, the invention may comprise an electrical circuit for the determination of the sign and magnitude of thev diierence between the frequencies of two waves comprising a single-sideband modulator in which the outputs of the two modulating paths are combined by means of a four-wire terminating set, and arranged so that a lower sideband is obtained from one pair of output terminals of the terminating set when the frequency difference has one sign, and from the other pair of output terminals of the terminating set conjugate to the rst mentioned pair when the frequency difference has the other sign.

Considered in terms of an embodiment, the invention may consist in an electrical circuit for producing a rectified current depending on the magnitude and sign of the diierence between the frequencies of two waves, comprising two pairs of modulators and two output rectiilers, means for applying the waves of the rst frequency in the same phase to one modulator of each pair and in the oppositeA phase to the other modulator of each pair, means for applying the waves of the second frequency in one phase to both modulators of one pair, and in a phase differing therefrom by ninety degrees to both modulators of the other pair, means for rseparately combining the outputs of the modulators of each pair, and means f or applying the sum of the combined outputs to one output rectifier and the difference to the other, one of the said combined outputs being rst changed in phase by ninety degrees.

Finally, in terms of another embodiment, the

invention may consist in an electricl circuit for producing a rectiiied current depending on the magnitude and sign of the diierence between the frequencies of two waves, comprising two modulators and two output rectiiiers, means for applying the waves of the first frequency to the modulators, means for applying the waves of the second frequency to the modulators respectively in phases differing by ninety degrees, means for applying the sidebands derived from one modulator to each output rectifier, and means for applying the sidebands derived from the other modulator to one output rectiiier advanced in phase by ninety degrees, and to the other output rectifier retarded in phase by ninety degrees.

The invention will be described with reference to the accompanying drawings in which- Figure l shows a block schematic diagram which exhibits the basic principles of the invention;

Figures 2 and 3 show modifications of Figure i;

Figures 4 and 6 show schematic circuit diagrams of two embodiments operating on the same principles as Figure 1; and- Figures and 7 show details of modulators used respectively in Figures 4 and 6.

The block schematic circuit diagram Figure 1 shows the principle of the invention. Currents having the frequencies F1 and F2 (either or both of which frequencies may be yariable) whose diierence f it is desired to determine are applied to two corresponding iilters I and 2, which are only required if there are likely to be any undesired interfering frequencies which should be removed.

After passing through the lter I the fre'- quency Fi is applied to two similar branches of the circuit in parallel. The upper branch comprises two similar modulators 3 and 5 connected in parallel, and leading to a quadrature phase difference network 1. This network comprises two phase changing networks connected respectively in tandem with the modulators 3 and 5 and designed to produce at all material frequencies a phase diierence of 90 between the two output currents," which are then combined and applied through a low pass filter 9 and shaping network II to an output rectifier or detector I3 having connected to its output a resistance Ri, across which it produces a continuous diierence of potential. Y

The lower branch of the circuit comprises similar elements, namely the modulators l and i, the quadrature phase difference network 8, filter III, shaping network I2 and detector Il connected to a second resistance Re equal to resistance R1.

The current of frequency Fn is used as a carrier current for the modulators 3i, 4, 5 and 6 and is supplied to them in pairs through a quadrature phase difference network i5 of the same kind as the networks l and 8, so that the modulators 5 and 6 are supplied with carrier current having a phase diiferlng by 90 from the phase of the carrier current supplied to the modulators 3 and 4. An indicating instrument I6 is connected to the extreme terminals of the resistances Ri and R2 as shown. The instrument ISmay, for example, be a centre zero direct current voltmeter. The networks 1, 8 and I5 may, for example, be designed in the manner described in the speciiication of application Serial No. 432,680, now' Patent y2,392,476 which issued on January 8, 1946, but any other convenient means for producing a phase difference of 90 over a frequency band 01 the necessary width may be used. The low pass iilters 8 and Ib are used to eliminate unwanted upper sidebands, ,and the shaping networks II and I2 are introduced in order that the voltages applied to the detectors Il and Il may be made to vary with the difference frequency f in any desired way; the method of designing these networks is well known. The detectors I3 and Il may be of any suitable type and the connections to the resistances Ri and R.: should be poled so that the direct currents iiow in oppostedlrections in R1 and Rz as indicated by the arrows in Fig. 1.

It may be pointed out that if the shaping networks II and I2 be omitted, the arrangement can j 1,666,206; and also in United States Patent No.

2,248,250, 4which shows various single-sideband modulation circuit arrangements by means of winch certain sidebands may be made substantially to disappear. In this specication any of these arrangements will be called a single-sideband modulator. According to the present invention, the arrangement is such that one branch of Figure 1 produces the lower sideband Fi-Fa only if F1 1?a, and the other branch produces the lower Sideband Fz-Fi only if F FL It will be assumed that each of the networks 1, 8 and I5 introduces a phase change of q: into the lower path, as indicated in Figure 1, and p4-90 into the upper path. The angle p may depend on the frequency, but does not aiect the cancelling of the sidebands; For explaining the invention, it will be assumed for simplicity that c is zero, so that each of the networks 1, 8 and I5 can then be regarded as consisting of a. single 90 phase changing `network arranged in the upper circuit only. The eiect of vp is merely to change the phase ofthe combined current output into the filters 8 and Il and this phase is immaterial since these currents are subsequently separately rectiiied.

By reference to United States Patent No. 2,248,250 it will be seen that the upper branch of Figure 1 of the accompanying drawings is substantially the same as Figure 2 of that patent when transmission is from right` to left. As shown therein, a lower sideband output Fi-Fa will be obtained only if F1 Fz and nothing at all if F1 Fz. Similarly, the lower branch of` the accompanying Figure 1 is substantially the same as Figure 1 of the United States Patent for transmission from right to left. The lower sideband Y, Fix-F1 is obtained onlyv when F: Fi, but is accompanied by the upper sideband Fi-i-Fz. If

also in the lower branch but not in the upper branch.

The nlters 9 and I0 are accordingly provided to eliminate the upper sidebands. Theoretically;

germs filter 9 is not required, but in practice the complete elimination of the upper sideband F1+Fz in the upper branch is difficult to realise. because it involves designing the network 1 to give a 90 phase difference over a very wide frequency range kwhich includes F14-Fa as well as l. It will thus be seen that a lower sideband input is supplied to the shaping network only when F1 F1 and to the other shaping network I2 only when F1 Fz. A rectified current is therefore obtained in'Rr proportional to the frequency difference f in the first case, and in Rz in the second case, so that the instrument I6 reads on one side of the zero when f is positive and on the other side when f is negative.

It should be explained that at the cost of an extra quadrature phase difference network, theoretical elimination of the upper sideband in both the branches can be obtained. 'Ihis is shown in Figure 2, which indicates the modification of the left-hand portion of Figure 1 to produce this resuit, everything to the right of the modulators I and 6 being the same asin Figure 1.

The extra network I8 may be similar to the network |5 and is poled the same way so that it introduces a phase change of 90+tp in the upper'.

path. Thev lower branch ofthe circuit is now similar to Figure 4 -of United States Patent No. 2,248,250, for transmission either way. Since, however, as already explained, the suppression of the upper sidebands in the modulators is never completely secured, the filters 9 and I 0 (Figure 1) would generally be needed in practice, so that the extra network I8 gives little if any practical advantage.

Referring again to Fig. 1, the two components which are combined at the output side of the network I are the same as those combined at the output of the network 8, except that output of network 8 includes in addition a frequency Ft-l-Fz but they are added in one case and "subtracted in the other; or in other words one of aircraft locating system in which an aeroplane pursuing another machine carries a transmitting aerial Z, emitting a carrier wave having a frequency F, and two receiving aerials X and Y placed at some distance apart. The receiving aerials pick up the waves reflected from the pursued machine. which waves will in general have frequencies slightly different from that transmitted and from each other on account of the Doppler effect. These frequencies will be denoted lll-F1 and F-l-Fz respectively, and F1 and F: will be small compared with F. By measuring the difference Fl-Fz in magnitude and sign it is possible to derive the necessary information to enable the pursuing machine to set the best course.

The circuit of Fig. 4 comprises two parts, the first of which is shown in block schematic form and illustrates one way in which waves having frequencies F1 and F: Amay be conveniently derived by known means from the waves-received separately by the aerials X and Y. The second part shows an arrangement according to the principles explained with reference to Fig. 1 for measuring Fx-Fz.

The block 2l in Fig. 4 represents the oscillator carried by the aeroplane for generating the waves F which are supplied to the transmitting aerial Z. 2|, 22 and 23 are three similar frequency changers of any suitable type supplied with waves at frequencyF from 2l.

0f these, frequency changers 2| and 22 are supplied direct, but frequency changer 23 is supplied through a network 3l which introduces a change of phase of 90. If the ciilator 2l is connected to frequency changer 23 by a cable tion being carried outby the extra length of cable. i

. The frequency changer 2| is connected to the the components is in the same phase at the out- 1.

puts of'networks .l and 8 while the other is in l opposite phases at these points. The recognition -of this circumstance enables Fig. 1 to be simplifled, as shown in Fig. 3. The modulators 3 and 5,

and the quadrature network 'l are not required.

The two components at the output of the net-v work 8 are applied respectively to two conjugate nating set TS of well known type. The other two conjugate pairs of output terminals of TS are respectively connected to the input sides of the filters 9 and l0, the remainder of the circuit vbeing as shown in Fig. 1. According to the well `known properties of a four-wire terminating set, the two components derived' from l will be added at the input of one of the filters 9, Il, and "subtracted at the input -of the other, so

that the effect on the instrument |6will be the same as in Fig. 1. The arrangement of Fig. 3

to identify separately allthe elements -shown in Figure 1. f

Figure 4 shows an example of one practical circuit according to thev invention, adapted for an 75 l output level.

pairs of input terminals of a four-wire termi- 50V aerial X, and frequency changers' 22 and 23 areV connected in'parallel to aerial Y. 2|, 25v and 2i are filters adapted to remove unwantedv frequencies-derived Vfrom the frequency changers, and 21,

2l and 29v are amplifiers, which are lpreferably provided with means It win Vbe that waveshvingf frequencies' rebut datering in phase'by 9o? winne obtained at the output ofthe ampliers 28and"29f; It will be assumed for simplicity that F2 'derived from amplifier '2l has zero-phase, so .that the other Fz hasY a phase of The Ft waves may'have any phe- The outputs of 2! are applied respectively to the primary .windings of three transformers Tc, Tia and Ta. 0f these transformers TA and Tra-have two equal secondary windings, and transformer Tc has four equal secondary windings.

The poling of these secondary windings 1s indicated in Figure 4 by and signs, which are intended to be interpreted as follows:

When an alternating'current is supplied to the primary winding of any transformer, 'the potentials at the ends of all windings of that transformer marked will be in the sam'e phase.

The measuring circuit comprises two valves V1 and Vzarranged as rectifiers in opposition, which -valves correspond to the Voutput rectiflers Hand w M in Figure 1.v The resistancesl Rrand` Rz, which correspond; to the Asame resistances shown in Figure 1, are connected respectively in-fseries with y l the anodes. the anodebattery B1, (or otherr suit.`

for maintaining a'constant thethree amplifiers 21, and 'i able high tension source) being connected to the junction point of these resistances. The instrument I8 is connected as shown in Figure 1, and is shunted by a suitable by-pass condenser C, for removing alternating current components from the rectified output 4of the valves.

Two equal resistances R and Rs are connected in series with a portion of the control grid circuit common to both valves and an appropriate biassing battery or other source B2 is also included. The two cathodes may be earthed as indicated.

Rv and Rs are equal resistances shunting the control grids, and the potential drop across two equal resistances, R3 and R4, is applied between the control grids through equal condensers Cv and Cs.

The resistances Ra. R4, R5 and Rs respectively form the loads of four similar modulators Ml, Mz, Ms and M4. Figure 5 shows the details of one of these modulators M, which comprises four rectiers (such as copper oxide or selenium rectifiers) arranged in a bridge of which one pair of diagonal corners are connected to the input terminals I, 2 and the other pair of diagonal corners are connected to the output terminals 3, d. These numbers are indicated in Figure 4 in order to deiine the relativepoling of the rectiiiers.

It will be seen that modulators M1 and Mz are supplied with frequency F: in the same phase but with frequency F1 in opposite phases, and similarLv for modulators bh and M4; but that the phase ol' Fb as supplied to modulators M1 and Mz is 90 in advance of the phase as supplied to modulators M3 and M4. Further, the outputs of both pairs of rectliers are respectively connected in series opposing.

The L type condenser-resistance networks C1, R1 and Cl, Ra perform the combined functions of elements I and Il, and 8 and I2 of Fig. 1, respectively. The resistances Rv and Rs should be small compared with the impedances of the condensers C1 and Csover the frequency range of interest. In that case, the voltage across resistances R1 and Rn will be advanced in phase by substantially 90 over the voltage across resistances R3 and R4 and moreover will be proportional to the frequency.

Hence the voltage applied to the control grids of V1 and Vz will be equal to the voltage drop across resistances Rs and Rs plus or minus, respectively, a voltage proportional to the drop across resistances R1 and R4 advanced in phase by 90.

'I'he networks C1 R1 and Ca Ra represent only one way of obtaining the desired phase change of 90. Similar results could be obtained, for instance, by means of a mutual inductance coupling.

The valves Vl and V2 are arranged effectively as a balanced modulator, The input voltages applied to the control grids in parallel, which are derived from the resistances R5 and Re. wouldA alone cause no deflection of the instrument I6, since the currents in resistances R1 and R2 would be equal and opposite. The eiect of the pushpull input voltage to the grids, which is derived from the resistances R1 and Rs. is to unbalance the voltages applied in parallel to the grids in a direction which depends on `the relative phases of the push-pull and parallel input voltages. 'I'he push-pull and parallel input voltages are in the same or in opposite phases depending upon whether frequencies F1 or F: is the larger. The unbalance current registered in the instrument I6 is proportional to Fl-Eb as a result of the use of the networks C7 R1 and Ca Ra so long as the ampliers 2l, 28 and 29 give constant output volume. If the circuit is required only as a null indicator (that is, to show when F1=F2) the ampliers need not give constant output, but some automatic output control will generally be desirable.

Valves V1 and V2 are effectively arranged as a balanced demodulator, the input across resistances R5 and Re (parallel input) causes no deflection of the meter I6 since the currents through resistances R1 and Rz tend to balance; the effect of the input across resistances R1 and Rs (push-pull input) is to imbalance the input to the valves in a direction which depends on the phase of the push-pull input voltage in relation to the parallel input voltage. The push-pull input voltage is of the same or of opposite phase to the parallel input voltage depending on whether friequency F1 or F2 is the larger. The unbalance current registered by the indicator I6 is proportional to Fi-Fz. This is obtained by using constant output amplifiers for ampliers 2l, 28 and 29 and by the use of the circuit Cv Cs and R1 Rs.

If the circuit is required mainly as a null indicator, the amplifiers 21, 28 and 29 need not be of the constant output type, but some automatic output control will generally be desirable. As explained in connection with Figure 1, unwanted sidebands Foi-F2 will appear. These may be eliminated by appropriate lters as mentioned. In the case of Figure 4, these sidebands are substantially eliminated =by means of the condensers C3, C4, C5 and Cs connected to shunt the load resistances Rs, R4, Rs and Re. This is quite satisfactory when F14-Fs is large compared with f. In other cases these condensers may be replaced by one or more sections of appropriate wave filters inserted `at the same points.

Alternatively, also condensers Ca and C4 may be omitted and a single condenser may be connected between the points H and K or a wave filter may be inserted at this point. Likewise condensers Cs and Ce may be omitted, and resistances R5 and Re may be together shunted by a single condenser, or they may be connected to the control grid circuit through a wave filter. None of these alternative condensers or filters are shown in Figure 4.

A simplified version of Figure 4 is shown in Figure 6, in which substantially the same results are obtained. Those elements in Figure 6 which are the same as those in Figure 4 have been given the same designations and need not be again described. Two ring modulators RM1 and RMz are used instead of four bridge rectiers, and transformers with tapped secondary windings are employed. The left-hand portion of Figure 4 in which the frequencies F1 and F2 are derived from the aerials is not reproduced in Figure G, but it will beunderstood that they can be obtained in this or in any other desired way.

The transformers TTA and TTB which supply the frequency F2 each have a centre tapped secondary winding, and the transformer 'I'Ic which supplied F1 has two similar centre-tapped secondary windings. The two ring modulators RM1 and RMz may have the circuit shown in Figure '1. Each of the ring modulators 'is supplied from one of the windings of transformer 'I'I`c, and ring modulator RM1 is also supplied from transformer TTA, and ring modulator RMz from transformer TTB. The two control grids are supplied in series from the secondary winding taps ol 2,474,ass

9 transformers TTA and '.tTc, and in parallel from the taps of transformers 'I'In and T'Ic.

The condensers Ca and Cm are included for the other means for performing some function to be -controlled by the frequency difference.

. It may be further pointed out that any convenient rectiers could be used instead of the` valves V1 and Vn, the circuit being appropriately modified.

What is claimed is:

1. In an electrical measuring circuit, a first A. C. source of one frequency, a second A. C. source of a frequency to be compared with the first frequency, means for modulating the first by the second of said frequencies, comprising two modulating branches, means for applying said two sources to each of said two branches, and means for combining the outputs of the two modulating branches to obtain a lower sideband from one pair of output terminals when the frequency difference has one sign, and from the other pair of output terminals when the frequency difference has the other sign.

2. In an electrical frequency comparison circuit, means for supplying waves of one frequency, means for supplying waves' of another frequency to be compared with said irst frequency, means for modulating said rst waves by said latter waves to produce a difference wave comprising two single-sideband modulators, and means for supplying each of said modulators with waves of both frequencies, the waves of one frequency being supplied to both modulators in phases diifering by ninety degrees respectively, the waves of the other frequency being supplied to both modulators in the same phase.

3. In an electrical frequency comparison circuit, means for supplying waves of one frequency, means for supplying waves of another frequency to be compared with said rst frequency, means for modulating said first waves by said latter waves to produce difference waves comprising two supplying a second frequency wave, two pairs of modulators and two output rectitlersmeans for applying the waves of the rst frequency in the same phase to one modulator of each pair and in the opposite phase to the other modulator of each pair, means for applying the waves of the second frequency in one phase to both modulators of one pair and in a phase diering therefrom by' ninety degrees to both modulators of the other pair, and means for separately combining the outputs of the modulators of each pair, and means for applying the sum of the combined outputs to one output rectifier and the difference to the other, and means for changing one of the said combined outputs in phase by ninety degrees before combining one with the other of said outputs.

8. In a circuit according to claim 7, means for causing the output current of each rectifier to be substantially proportional to the difference frequency.

9. In a circuit according to claim 7, means for substantially eliminating the upperv sidebands' from the outputs of the modulators.

10. In a circuit according tov claim '1, means for substantially eliminating the upper sidebands from the outputs of the modulators including a condenser shunting the output of each modulator.

' 11. In a circuit according to claim 3, means for combining the outputs having said different phases and applyingl the result to rectifying means including a bridge rectifier for each of said modulators.

12. In an electrical comparison circuit,V means for supplying a irst wave of known frequency, means for supplyingA a second wave of a frequency to be compared with said known frequency, means for modulating the rst by the second wave, two modulators and two output rectiiers, means for applying the rst wave to the two modulators, means for applying the second wave to the modulators respectively in phases differing by ninety degrees, means for applying the sidebands derived from one modulator to each output rectifier, and means for applying the sidebands derived from the other modulator to' one output rectifier advanced in phase by'ninety degrees, and to the other output rectifier retarded in phase by ninety degrees.

, 13. In a circuit according to claim 12, a common transformer having two center-tapped secondary windings and supplying both modulators with the rst wave, and individual transformers each having a center-tapped secondary winding single-sideband modulators, means for supplying each of said modulators with waves of both frephase shifting the output of each of said modulators to produce a relative phase difference of ninety degrees.

4. In a circuit according to claim 3, means for combining the outputs having different phases and applying the result to rectifying means.

5. In a circuit according to claim 3, means for combining the outputs having said different phases, applying the result to rectifying means, and network means for shaping said result to make the rectified outputs dependent upon the frequency. 6. In a circuit according to claim 3, means for combining the outputs having said different phases, applying the result to rectifying means, and means for supplying said result to a common utilization device.

7. In an electrical measuring circuit, means for supplying a rst frequency wave, means for quencies to obtain lower sidebands and means for and supplying each modulator with the second wave.

14. In a circuit according to claim 12, a common transformer having two center-tapped secondary windings and supplying both modulators with the first wave, and individual transformers each having a center-tapped secondary winding and supplying each modulator with the second wave, in which the sidebands derived from each modulator are obtained from the center-taps of the corresponding secondary windings.

15-111 a crcuit according to claim 12, a common transformer having two center-tapped secondary windings and supplying both modulators with the iirst wave,and individual transformers each having a center-tapped secondary winding and supplying each'modulator with the second wave, and means for substantially eliminating the upper sidebands derived from the two modulators.

1s. 1n a circuit according to claim 12, a common transformer having two center-tapped secondary windings and supplying both modulators with the first wave, and individual transformers lators including a condenser connected between the center taps corresponding to each modulator.

17. In a circuit according to claim 12, a fourrectier bridge circuit.

18. In a circuit according to claim 12, a common transformer having two center-tapped secondary windings and supplying both modulators with the rst wave, individual transformers each having a center-tapped secondary winding and supplying each modulator with the second wave, and means for supplying the rectified outputs to two resistances sociated with each output and connected in series to a utilization device.

19. In a circuit according to claim 12, a common transformer having two centertapped secondary windings and supplying both modulators with the first wave, individual transformers each having a center-tapped secondary winding and supplying each modulator with the second wave, and means for supplying the rectified outputs to two resistances associated with each output and connected in series to a utilization device consisting of a direct current indicating instrument.

20. In a circuit according to claim 12, output rectifier means including thermionic valves having biasing means. a common transformer having two center-tapped secondary windings and supplying both modulators with the flrst wave, and individual transformers each having a center-tapped secondary winding and supplying each modulator with the second wave.

21. An electrical frequency comparing circuit comprising a first source of electrical oscillations,

a second source of electrical oscillations at a frequency different from the frequency of said rst source, a pair of channels, means to apply the frequencies of said sources to each of said channels, means in one of said channels to produce a direct current output when the frequency of said rst source is greater than the frequency of said second source and to produce a greatly reduced output when the freqeuncy oi said first source is less than the frequency of said second source. means in said other channel to produce a direct current output when the frequency of said second source is greater than that of said rst source and to produce a greatly reduced output when the frequency of said second source is less than that of said rst source, and means to indicate which of said channels has the greater output. A

22. An electrical frequency comparing circuit in accordance with claim 21 in which means is provided in each channel to control the magnitude of the output thereof-in accordance with the difference between the frequencies of said sources.

23. An electrical frequency comparing circuit in accordance with claim 21 in which the means in each channel to produce the direct current output comprises a single side band modulator.

BENT BULOW JACOBSEN.

REFERENCES CITED 'Ihe following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,931,873 Marrison Oct. 24, 1933 2,176,742 La Pierre Oct. 17, 1939 2,207,540 Hansel July 9, 1940 2,220,098 Guanella Nov. 5, 1940 2,248,250 Peterson July 8, 1941 

